1. Field of the Invention
The present invention relates to a scanning optical apparatus and, more particularly, to a scanning optical apparatus suitably applicable to devices arranged to deflect and reflect light optically modulated and emitted from light source means, by an optical deflector comprised of a rotary polygon mirror, etc., and thereafter to make the light scan an area on a scanned surface through an imaging optical system (f.theta. lens) having the f.theta. characteristic to record image information thereon, for example, such as laser beam printer apparatus (LBP) or digital copiers having the electrophotographic process.
2. Related Background Art
In the conventional scanning optical apparatus such as the laser beam printers, the beam optically modulated according to an image signal and emitted from the light source means is regularly deflected by the optical deflector, for example, comprised of a rotary polygon mirror (polygon mirror), the beam is converged in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the imaging optical system having the fe characteristic, and the beam is made to optically scan the area on the surface to undergo image recording.
FIG. 1 is a schematic drawing to show the major part of a conventional scanning optical apparatus, which is a cross--sectional view thereof in the main scanning direction. In the drawing diverging light emitted from the light source means 11 is converted into a nearly parallel beam by collimator lens 12, stop 13 limits the beam (quantity of light), and then the beam is incident to cylindrical lens 14 which has predetermined refracting power only in the sub scanning direction. The parallel beam incident to the cylindrical lens 14 is emergent in the state of parallel rays as it is, in the main scanning section. In the sub scanning section the beam is converged to be focused as a nearly linear image on a deflective surface (reflective surface) 15a of the optical deflector 15 comprised of the rotary polygon mirror (polygon mirror).
Then the beam deflected by the deflective surface 15a of optical deflector 15 is guided through the imaging optical system (f.theta. lens) 16 having the f.theta. characteristic onto the surface of photosensitive drum 18 as a scanned surface and the beam is made to optically scan the area on the surface of photosensitive drum 18 by rotating the optical deflector 15 in the direction of arrow A, thereby performing recording of image information.
In order to undergo highly accurate recording of image information, the scanning optical apparatus of this type needs to be well corrected for curvature of field throughout the entire region of the scanned surface so as to have uniform spot diameters and needs to have distortion (the f.theta. characteristic) for achieving the proportional relation between angle of incident light and height of image. A variety of scanning optical devices or correction optical systems (f.theta. lenses) thereof satisfying such optical characteristics have been proposed heretofore.
On the other hand, as the scale and cost of the laser beam printers, digital copiers, etc. are decreasing, the scanning optical apparatus is also required to decrease the same.
For simultaneously meeting these demands, various scanning optical devices comprised of a single f.theta. lens were proposed, for example, in Japanese Patent Publication No. 61-48684, Japanese Laid-open Patent Application No. 63-157122, Japanese Laid-open Patent Application No. 4-104213, Japanese Laid-open Patent Application No. 4-50908 (corresponding to U.S. Pat. No. 5,111,219), and so on.
Japanese Patent Publication No. 61-48684, Japanese Laid-open Patent Application No. 63-157122, etc., out of these patents and applications, disclose that the parallel beam from the collimator lens is converged on the surface of recording medium by use of a single lens with a concave surface on the optical deflector side as an f.theta. lens. Further, Japanese Laid-open Patent Application No. 4-104213 discloses that a single lens with a concave surface on the optical deflector side and a toroidal surface on the image plane side is used as an f.theta. lens and a beam, which was converted into converging light by the collimator lens, is made incident to the f.theta. lens. Japanese Laid-open Patent Application No. 4-50908 (corresponding to U.S. Pat. No. 5,111,219) discloses that a single lens with lens surfaces being higher-order aspherical surfaces is used as an f.theta. lens and the beam, which was converted into the converging light by the collimator lens, is made incident to the f.theta. lens.
The conventional scanning optical devices as described above, however, had the following problems. In the apparatus of Japanese Patent Publication No. 61-48684, the curvature of field remains in the sub scanning direction; and, because the parallel beam is focused on the scanned surface, the focal length f, a distance from the f.theta. lens to the scanned surface, is long, thus posing the problem that it is difficult to construct a compact scanning optical device.
The apparatus of Japanese Laid-open Patent Application No. 63-157122 had the problem that it was difficult to produce the fO lens by molding because the thickness thereof was large and it was the cause of increase in the cost.
The apparatus of Japanese Laid-open Patent Application No. 4-104213 had the problems that distortion remained and that jitter occurred in the period of the number of polygon faces because of a mounting error of the polygon mirror being the optical deflector.
In the apparatus of Japanese Laid-open Patent Application No. 4-50908, aberration was well corrected for by use of the f.theta. lens of higher-order aspherical surfaces, whereas spot diameters in the sub scanning direction tended to vary depending upon heights of image because of nonuniformity of magnification in the sub scanning direction between the optical deflector and the scanned surface.
In addition to the above, the scanning optical devices with the f.theta. lens comprised of two lenses were proposed, for example, in Japanese Laid-open Patent Application No. 56-36622, Japanese Laid-open Patent Application No. 61-175607, and so on. The f.theta. lenses proposed in these were constructed in a cross-sectional shape composed of spherical surfaces or weak aspherical surfaces, and with these f.theta. lenses it seemed rather difficult to decrease the scale, decrease the cost, and increase the definition.